If master server goes down - the slave is get his functions after failture?
Or in this case we need to reconfigure the router to route ports to another IP?
If it is, can we use a heartbeat package with ISPConfig3?

Are you are sure that you executed the install.php script and not the update.php script on srv2? The error message that you posted exists in the updater only, not the installersoit can not appear during installation.

The above error is not related to creating a client in ispconfig. The query is from a ispconfig update were the ispconfig datannbase must have been updated or manually replaced before so that the updater could not recognize its structure due to a missing db version number in the server table of this server. So it is something related to your initial setup, maybe you installed ispconfig more then once on the server without removing it completely before.

The mirror guide installs a bidirectional mirroring. So it oes not matter if sou crate a mailbox on the master or the slave as the settings are applied on both servers automatically. At the end of the guide are instructions to copy the config.inc.php file from master to slave, this step is important ans has to be redone after an update to ensure that sou can use the ispconfig interface on the master and the slave.